A few things occurred to me reading this thread. First, does definitely sound as though a vet visit is in order to answer some basic questions. Do make sure your vet knows about rabbits! Wonderful cat and dog vets can do their best and still make awful mistakes when looking at or treating a rabbit. There is a list on the rabbit.org website of vets, and yes there are some in Ontario. There are several in Toronto and Ottawa (other places too but those didn’t stick in my mind) so if there isn’t one close to you an idea might be to phone one in the nearest large city and ask if they could recommend any vets closer to you. The nearest SPCA that has an exotic pet facility would also have information on local vets. That doesn’t mean there may be one close by, the closest options I have are Montreal and Ottawa, and I’m about dead centre between them, a minimum hour drive either way.
Secondly, he hasn’t been with you long and you have been making major (and wonderful) changes to his diet so some upset in his system is probably to be expected. I’d still set up that vet visit but I wouldn’t get really scared.
A seven month old rabbit could breed, … I don’t know if a breeder would use them that young but I know it is physically possible. However, doesn’t sound as though you were given much reliable information.
The absolute best way to get more fibre in his diet is to give him hay, and lots of hay! There are different kinds, timothy’s is a staple, but orchard and meadow are just two of the other kinds available. You can try mixing a few kinds for starters, and make sure he always has plenty in his rack etc. No matter what age he is having more different kinds of hay will be good for him. I’d think it would be best to be cautious, he’s probably an adult so simply avoid all alfalfa, dried, and pellets. Martin’s original adult is an excellent timothy based pellet. Many people feed Oxbo, they are roughly equivalent. (If you do go with Martins’ make sure it is the original, because they also have an alfalfa based pellet.) Up to you of course but I’d start phasing him off of those pellets, and wouldn’t worry too much about whether or not it was what the breeder used to feed him. Sorry about the waste, that never feels good but really his health is the bigger issue. But the more hay he eats the better.
And yes, pet shops are Not the best place to get info, … even for dogs and cats (although they are better about those) they still sell toys that are dangerous and food that is bad. Much better to check the HRS’ site, search on this forum, and if in doubt ask!
Oh, and sorry but humping is a dominence thing, he may stop doing it, or do it less if he is fixed, but he may keep right on doing it since he’s proving to your cat, and you, and lots of other things that He’s Top Rabbit!
Let us know how things go.